This invention relates generally to a highlight color imaging and more particularly to a multi-roll developer housing with converging belt to roll spacing.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. Only the imaging area of the photoreceptor is uniformly charged. The image area does not extend across the entire width of the photoreceptor. Accordingly, the edges of the photoreceptor are not charged. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged areas) of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor. The charge areas are developed using Charged Area Development (CAD) while the discharged areas are developed using Discharged Area Development (DAD).
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the charge pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge.
As will be appreciated, the quality of the prints produced through the use of a magnetic brush development process are dependent on the spacing of the development roll (BRS) or rolls from the photoreceptor. Generally, an electric field is established between those rolls and a supporting substrate for the photoreceptor to suppress background development. Thus, any change in the development roll spacing not only alters the quantity of toner deposited on the image, but also varies the gradient field. It goes without saying that changes in either of those two parameters are necessarily accompanied by corresponding changes in the quality of the prints produced. In addition to print quality issues in respect to the spacing of the development rolls from the photoreceptor, it has been found by the Applicants that machine problems occur which are also affected by the spacing of the development rolls from the photoreceptor, such as developer housing cross-contamination, electrostatic voltmeter (ESV) contamination and other machine dirt related problems.
Therefore, it is highly desirable to determine the optimum development roll-to-photoreceptor spacing which will alleviate the machine problems while maintaining high quality copies.